Since the recent advent of several exciting new ceramic materials useful for microelectronics, primarily as an insulating medium or support substrate thereof, there has developed an urgent need for the provision of electroconductive materials which are compatible therewith, both as to thermal expansion coefficient and thermal conductivity matching. The new ceramics mentioned are aluminum nitride, silicon carbide and boron nitride, which exhibit properties that are comparable with, and in some instances superior, to alumina and beryllium oxide. Therefore, these new ceramics are currently under intense test and evaluation for military and commercial uses as substitutes for the prior art ceramics used for electronic circuit substrates. In the particular case of aluminum nitride, it is recognized and considered by some authorities in the field of ceramics, as equal to or superior to byrellium oxide as a microelectronic substrate material, see Ceramic Industry, May 1989, pages 25-27.
The need for improved metal clad-coated refractory particles arises from the fact that such particles are thought to possess both good electrical and metallurgical properties and are not readily available in a wide range of desired combinations. This is especially so in particle sizes of less than 10 microns and particles having prescribed percent of coating material by weight to the core of the particle material. Prior art technology techniques for coating of particles are principally by means of chemical vapor deposition or electroless plating and such processes are not readily adaptable to controlled deposition of a wide range of coating materials and for micron size particles. In addition, when such prior art techniques are employed for coating purposes, it is recognized that there are often residual chemical contaminates which are detrimental to thick-film pastes and coatings used for electronic circuits, and radiofrequency and elctromagnetic shielding.
In the case of metal matrix composites utilizing refractory metals, considerable time, research and monies have been expended to provide composites which exhibit improved mechanical properties. In this connection, a well recognized technique known as rapid solidification processing have been used in efforts to provide the improved mechanical properties desired. The resultant amorphous particle contains precipitates which behave as a dispersed second phase. However this metastable structure recrystallizes at high temperatures resulting in the loss of any mechanical strengthening benefits. In addition, the technology is limited to specific compositions that limit the actual metal content of the particles.
In the prior art one notable method for the coating of particles is the carbonyl process, where under certain conditions, carbon monoxide combines with metal to form a volatile metal carbonyl. When the metal carbonyl is heated to temperatures above 200 deg C at atmospheric pressure, the reverse reaction takes place, releasing carbon monoxide and depositing pure metal. When metal carbonyl is fed into a heated bed of fluidized particles, for example tungsten particles, it decomposes depositing metal on the particles. See U.S. Pat. No. 3,342,587, issued Sept. 19, 1967, to C. B. Goodrich, et. al. In theory, the carbonyl process would appear to provide a wide range of opportunities to coating various material particles both metallic and non-metallic.
However, in practice the process has proven to be limited as to the composition of materials and particle sizes, and therefore is not as universal, cost effective and as easy to implement as desired. See Modern Developments in Powder Metallurgy, Volumes 18-21, 1988, Metal Powder Industries Federation, Princeton, N.J. Nickel Coated Powders via the Carbonyl Nickel Gas Process, by E. L. Rees, et. al., Novamet Specialty Products, Corp., Wyckoff, N.J.
Another reason for seeking new techniques for combining refractory metals with traditional electroconductive metals such as copper, nickel, silver, gold or combinations thereof arises from the fact that binary alloys of refractory metals such as tungsten and molybdenum, as examples, are not readily formed and certainly are not cost effective in production of micron size particles. Similarly, ceramic materials are not known to combine readily as alloys with transition or other metals.
Finally, when the refractory metals are used as electroconductive filler for thick-films in microelectronic applications or in certain metal composite systems it is not necessary for the refractory metal core to be heated to the molten phase so long as the coating material reaches the molten or the near molten phase necessary for sintering. Therefore, pastes made with these materials which are screened onto green (unfired) ceramic tape can be made compatible with low temperature (i.e., less than 1000 deg C) cofired tape processes developed by companies such as IBM, DuPont and AT&T.
In the case of ceramic particles, a new and useful metal-to-ceramic composite system may be formed wherein clad-coated material reaches the molten or near molten phase during processing to thereby produce a high ceramic content in the metal-to-ceramic system which has advantages over an all metal system.